In recent years, power semiconductor devices have been applied to general industries and electric railroads, as well as have been mounted on vehicles, in view of environment issues. In particular, the power semiconductor devices mounted on vehicles are required to have high reliability in a limited space, and to achieve, at a low cost, an added value of improved fuel efficiency by mounting a motor and an inverter as compared with a conventional engine drive system. Specifically, a water-cooling power semiconductor device, which is typically mounted in an engine compartment, is required to achieve heat resistance and vibration resistance in the engine compartment as well as reduction in size. One of internal components of a power semiconductor device that account for a major proportion of cost is a semiconductor element. Accordingly, cost reduction is achieved, in the production of semiconductor elements, through reduction in loss of semiconductor elements, or reduction in size of the semiconductor elements to increase the number of semiconductor elements obtained from a wafer through enhancement of an operable temperature range by improving heat dissipation structure.
A power semiconductor device includes a semiconductor element mainly made of Si (linear expansion coefficient: 2.5 ppm/K) and external wiring mainly made of Cu (linear expansion coefficient: 16.8 ppm/K), Al (linear expansion coefficient: 23.0 ppm/K), and the like. The power semiconductor element has a product life determined by reliability of connection between electrodes on upper and lower surfaces of the semiconductor element and the external wiring.
The semiconductor element includes the lower surface electrode connected by soldering or the like, and reliability improvement through material improvement is reaching a limit. Recently considered is application of joining or the like by a CuSn compound, by Ag sintering, by Cu sintering, and the like, which achieve higher strength than solder.
On the other hand, the upper surface electrode of the semiconductor element is conventionally connected by Al wire bond joining to achieve required current capacity and connection reliability. However, the reduction in size of semiconductor elements causes increase in current density per unit volume of a connected portion, which may undesirably lead to deterioration in connection reliability.